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diff --git a/tools/nixery/docs/src/under-the-hood.md b/tools/nixery/docs/src/under-the-hood.md deleted file mode 100644 index 4b79830010..0000000000 --- a/tools/nixery/docs/src/under-the-hood.md +++ /dev/null @@ -1,129 +0,0 @@ -# Under the hood - -This page serves as a quick explanation of what happens under-the-hood when an -image is requested from Nixery. - -<!-- markdown-toc start - Don't edit this section. Run M-x markdown-toc-refresh-toc --> - -- [1. The image manifest is requested](#1-the-image-manifest-is-requested) -- [2. Nix fetches and prepares image content](#2-nix-fetches-and-prepares-image-content) -- [3. Layers are grouped, created, hashed, and persisted](#3-layers-are-grouped-created-hashed-and-persisted) -- [4. The manifest is assembled and returned to the client](#4-the-manifest-is-assembled-and-returned-to-the-client) -- [5. Image layers are requested](#5-image-layers-are-requested) - -<!-- markdown-toc end --> - --------- - -## 1. The image manifest is requested - -When container registry clients such as Docker pull an image, the first thing -they do is ask for the image manifest. This is a JSON document describing which -layers are contained in an image, as well as some additional auxiliary -information. - -This request is of the form `GET /v2/$imageName/manifests/$imageTag`. - -Nixery receives this request and begins by splitting the image name into its -path components and substituting meta-packages (such as `shell`) for their -contents. - -For example, requesting `shell/htop/git` results in Nixery expanding the image -name to `["bashInteractive", "coreutils", "htop", "git"]`. - -If Nixery is configured with a private Nix repository, it also looks at the -image tag and substitutes `latest` with `master`. - -It then invokes Nix with three parameters: - -1. image contents (as above) -2. image tag -3. configured package set source - -## 2. Nix fetches and prepares image content - -Using the parameters above, Nix imports the package set and begins by mapping -the image names to attributes in the package set. - -A special case during this process is packages with uppercase characters in -their name, for example anything under `haskellPackages`. The registry protocol -does not allow uppercase characters, so the Nix code will translate something -like `haskellpackages` (lowercased) to the correct attribute name. - -After identifying all contents, Nix uses the `symlinkJoin` function to -create a special layer with the "symlink farm" required to let the -image function like a normal disk image. - -Nix then returns information about the image contents as well as the -location of the special layer to Nixery. - -## 3. Layers are grouped, created, hashed, and persisted - -With the information received from Nix, Nixery determines the contents -of each layer while optimising for the best possible cache efficiency -(see the [layering design doc][] for details). - -With the grouped layers, Nixery then begins to create compressed -tarballs with all required contents for each layer. As these tarballs -are being created, they are simultaneously being hashed (as the image -manifest must contain the content-hashes of all layers) and persisted -to storage. - -Storage can be either a remote [Google Cloud Storage][gcs] bucket, or -a local filesystem path. - -During this step, Nixery checks its build cache (see [Caching][]) to -determine whether a layer needs to be built or is already cached from -a previous build. - -*Note:* While this step is running (which can take some time in the case of -large first-time image builds), the registry client is left hanging waiting for -an HTTP response. Unfortunately the registry protocol does not allow for any -feedback back to the user at this point, so from the user's perspective things -just ... hang, for a moment. - -## 4. The manifest is assembled and returned to the client - -Once armed with the hashes of all required layers, Nixery assembles -the OCI Container Image manifest which describes the structure of the -built image and names all of its layers by their content hash. - -This manifest is returned to the client. - -## 5. Image layers are requested - -The client now inspects the manifest and determines which of the -layers it is currently missing based on their content hashes. Note -that different container runtimes will handle this differently, and in -the case of certain engine and storage driver combinations (e.g. -Docker with OverlayFS) layers might be downloaded again even if they -are already present. - -For each of the missing layers, the client now issues a request to -Nixery that looks like this: - -`GET /v2/${imageName}/blob/sha256:${layerHash}` - -Nixery receives these requests and handles them based on the -configured storage backend. - -If the storage backend is GCS, it *redirects* them to Google Cloud -Storage URLs, responding with an `HTTP 303 See Other` status code and -the actual download URL of the layer. - -Nixery supports using private buckets which are not generally world-readable, in -which case [signed URLs][] are constructed using a private key. These allow the -registry client to download each layer without needing to care about how the -underlying authentication works. - -If the storage backend is the local filesystem, Nixery will attempt to -serve the layer back to the client from disk. - ---------- - -That's it. After these five steps the registry client has retrieved all it needs -to run the image produced by Nixery. - -[gcs]: https://cloud.google.com/storage/ -[signed URLs]: https://cloud.google.com/storage/docs/access-control/signed-urls -[layering design doc]: https://storage.googleapis.com/nixdoc/nixery-layers.html |